1. Field of the Invention
The present invention relates to asphalt paving compositions comprising a hydrated phosphate modifier, and more particularly to the inclusion of a hydrated phosphate modifier because it tends to decrease the temperature typically necessary for mixing and forming or compressing asphalt concrete to attain certain specifications and characteristics.
2. Description of the Related Technology
As is well known, asphalt is commonly used as a paving material. Typically, the asphalt, often referred to as “asphalt cement” or “asphalt binder,” is mixed with an aggregate to form an asphalt concrete suitable for paving. Thus, asphalt concrete is usually described as comprising aggregate held within a continuous phase of asphalt binder by adherence of the asphalt binder to the aggregate. Conventional methods of producing and paving with asphalt concrete typically require mixing the asphalt binder and aggregate at a temperature of at least 300° F. (about 150° C.) and paving at temperatures between about 265 and 320° F. (about 130 and 160° C.). Asphalt paving compositions made by such methods are often referred to as “hot mix asphalt” or “HMA” as used herein. In terms of handling, hot mix asphalt is able to be paved to the technical specifications and mechanical properties required by many governmental agencies. Unfortunately, however, the high temperatures typically used during production and paving of hot mix asphalt creates potential problems involving environmental, economic, and health concerns. For example, heating the components to typical hot mix temperatures requires a large amount of energy usually from burning of fossil fuels, which tends to be costly, generates emissions, including CO2 and noxious gases, and releases offensive odors.
Asphalt paving compositions may also be produced at temperatures lower than those used for hot mix asphalt (see, e.g., WO95/22661), and are often referred to as “cold mix asphalt.” This type of asphalt concrete, may be made by first preparing the asphalt binder as an emulsion, in which the liquid asphalt is suspended in water, or as a foam The asphalt binder emulsion or foam is then mixed with a cold and moist aggregate (“moist” is a relative term, it is aggregate that has not been heated to dry off moisture as in the hot mix process). Cold mix asphalt may be desirable for use where conventional asphalt would be difficult to produce, e.g., in remote locations or low ambient temperature environments. However, because of the methods used to make cold mix asphalt, it typically does not have the same characteristics as conventional hot mix asphalt. Cold mix asphalt tends to be permeable to water and air, which can make it more susceptible to a loss of road surface than hot mix asphalt. Cold mix asphalt also tends to have less cohesion than hot mix asphalt, which tends to result in less internal stability. Without being held to a particular theory, it is believed that cold mix asphalt suffers from these problems for several reasons related to the low temperatures employed in its production, including poor mixing of the binder and the aggregate, the presence of water in the asphalt concrete, and difficulty in spreading and compacting the asphalt concrete. Moreover, cold mix asphalt tends to require a longer cure time than hot mix asphalt, which is usually ready for use after the pavement has cooled. Despite the benefits from using lower temperatures, the use of cold mix asphalt has been limited because it typically cannot be used to produce a compacted asphalt concrete with technical specifications and mechanical properties substantially equivalent to that of a compacted hot mix asphalt.
Additionally, asphalt concrete may be produced at reduced temperatures between about 250 and about 275° F. (about 121 and about 135° C.), slightly lower than those used for making hot mix asphalt. Asphalt concrete made by such methods is often referred to as “warm mix asphalt” or “WMA” as used herein. Because warm mix asphalt is produced using lower temperatures than hot mix asphalt, it has the potential to be a lower-cost, lower-impact alternative to hot mix asphalt. That said, making warm mix asphalt typically requires some process modification(s) from that used to make hot mix asphalt so that the warm mix asphalt, when compressed, can have physical properties similar to that of compressed hot mix asphalt. For example, because of the reduced temperatures the asphalt binder tends to be more viscous, which can make it more difficult to mix, spread, and compact. To counter the increased viscosity typically associated with warm mix asphalt, a non-polymeric additive often referred to as a “warm mix modifier” or “warm mix additive” is typically added either directly to the asphalt mixture or to the asphalt binder before creating the asphalt mixture. Warm mix asphalt compositions typically do not require any other formula changes relative to hot mix asphalts and tend to have comparable resiliencies. Advantageously, it has been observed that for a variety of asphalt binders warm mixed asphalts tend to have increased densification/compaction across the ranges of usual temperatures and aggregate sizes.
Warm mix asphalt may offer several advantages for construction in the field. Because methods of making warm mix asphalt employ lower temperatures than conventional hot mix asphalt, it is safer to handle. Lower temperatures for warm mix asphalt may also allow a longer time period between picking up asphalt from a plant and laying the asphalt. Thus, hauling loads of asphalt over longer distances may be possible without a critical loss in temperature. When paving in cool ambient temperatures, warm mix asphalt tends to be easier to compact than hot mix asphalt because it tends to remain workable at lower temperatures and typically provides a longer window of time for compaction. As a result, warm mix asphalt is particularly suitable for paving in cool climates and can extend the paving season into the fall or winter in regions with moderate climates. This feature may be advantageous on large paving projects or projects with fall deadlines. Additionally, warm asphalt mixes may allow faster construction of pavements made up of deep lifts of asphalt because the temperature of warm asphalt mix is lower and less time is required to cool the mix between placing lifts. This is particularly advantageous for paving intersections, which generally need to be opened as soon as possible. Warm mix asphalt may be used for all lifts, usually about 0.75- to about 3-inches thick.
Warm mix technology is also compatible with common mix designs on the market, including Superpave and Marshall designs, as well as with production equipment and laydown equipment. Warm mix additives are also compatible with a wide ranges of aggregates and binders, including polymer-modified binders and binders comprising recycled asphalt. Additionally, in situations in which a stiff asphalt is desired polymeric modifiers are typically added, which tend to make achieving the desired degree of compaction more difficult, using warm mixes instead of hot mixes have helped achieve desired levels of compaction.
In addition to its advantages in construction, warm mix asphalt may also provide environmental, economic, and health benefits related to its use of temperatures below those used for conventional hot mix asphalt. Using reduced temperatures requires less energy and therefore consumes less fuel, typically fossil fuels. Thus, warm mix asphalt production has the potential benefits of using fewer non-renewable resources and controlling costs for producers of asphalt. Several warm mix technologies claim to reduce fuel consumption by 30 to 55%. Additionally, emission reductions has become an increasingly important concern especially in densely populated areas that persistently exceed governmental standards. Such regions have ozone levels high enough that the Environmental Protection Agency has designated them non-attainment areas. In these areas, emissions are highly regulated, so that asphalt production may be constrained by certain regulations, e.g., making asphalt mix at night. Because warm mix asphalt has the potential to reduce plant emissions in different stages of production and by up to 30%, its production may make it easier to obtain permits that would allow producers to make asphalt mix in areas and at times they previously could not. Emissions reduced by warm mix technologies include carbon dioxide, and carbon monoxide, sulfur dioxide, and nitrous oxides. Economically, reduced emissions may also reduce costs attributed to emissions control which can account for 30 to 50% of a producer's overhead costs. In addition to emissions, conventional asphalt heated to high temperatures used for making it also generates noxious odors and gases which may pose health risks for construction workers in the field. Using warm mix asphalt reduces the exposure of construction workers to these occupational hazards, as well as allowing safer handling at reduced temperatures.
In view of the foregoing, numerous efforts have been made to modify warm mix asphalt to increase its use. In particular, much of the effort has been directed to reducing the viscosity of asphalt by including various non-polymer modifier additives to the asphalt concrete mix or even to the binder compositions and the aggregate. The two primary types of modifying paving asphalt involve either foaming the asphalt or adding waxes (hydrocarbon-based modifiers) to the asphalt. Foaming methods are based on the principle of adding moisture to the asphalt and vaporizing the moisture to thereby cause the asphalt to foam, which tends to decrease the viscosity of the mix and improve adhesion. Waxes, on the other hand, are believed to improve the flow of the mix because of a lubricating effect.
One example of a foaming-type of technology involves the use of a zeolite, which is a hydrated silicate, as a warm mix asphalt modifier as disclosed in US 2005/0076810. Zeolites are framework silicates that have large interconnected vacant spaces and channels in their structures that allow easy absorption and release of water without damaging their structures. The percentage of water held internally by the zeolite is typically about 20% by mass and is usually released in the temperature range of 185° to 360° F. (about 185° to 182° C.). One such commercially available zeolite warm mix modifier is Aspha-Min® (Eurovia Services of Bottrop, Germany). It is a manufactured synthetic zeolite comprising sodium aluminum silicate that has been hydro thermally crystallized. Typically, Powdered Aspha-Min® is added to the asphalt mix at about 0.3% by mass of the mix at the same time as the binder. The heat associated with mixing asphalt drives water from the zeolite structure creating a volume expansion of the binder and resulting in foaming of the asphalt, which tends to increase the workability and aggregate coating to levels achieved at temperatures about 50° F. (30° C.) higher if the zeolite had not been added.
Another example of foaming-type technology involves using a two-component binder system for making warm mix asphalt as disclosed in U.S. Pat. No. 6,846,354. This technology is commercially available under the WAM-Foam® tradename by Shell International Petroleum, London, UK and Kolo-Veidekke, Oslo, Norway. In particular, using this technology the aggregate is first mixed with a soft asphalt binder at a temperature between about 230 and 250° F. (about 110° to 121° C.) and then the aggregate pre-coated with the soft binder is mixed with hard binder that is foamed by a concurrent injection of water that vaporizes. The foamed hard binder combines with the soft binder to achieve the required final composition and properties of the asphalt product. The initial coating of the aggregate with the soft binder is critical for preventing water from reaching and entering the aggregate. In fact, because moist aggregate negatively impacts adhesion of the binder thereto an adhesion improver is often used in the first mixing stage to enhance coating of the aggregate by the soft binder. The quality of the warm mix asphalt made using the WAM-Foam® technology also depends on the careful selection of the soft and hard binders.
Examples of asphalt modification waxes include Sasobit® (Sasol Wax, South Africa), which is fine crystalline, long chain aliphatic hydrocarbon produced from coal gasification using the Fischer-Tropsch (FT) process, and Asphaltan B® (Romonta GmbH, Amsdorf, Germany), which is a low molecular weight esterified wax based on Montan wax constituents and higher molecular weight hydrocarbons. Typically, in warm mix asphalt applications wax modifiers constitute between about 2 and 4% by weight of the mix.
Although many of the foregoing methods of reducing the temperature for making an asphalt concrete or paving composition that has mechanical properties similar to conventional hot mix asphalt have been effective to various degrees, a need continues to exist for a warm mix asphalt concrete or paving composition having mechanical properties similar to hot mix asphalt (e.g., degree of compaction and adhesion and resistance to stripping, fatigue, resistance to rutting, cracking, oxidation, etc.), in addition to other qualities that make it a desirable paving material (e.g., cost, energy requirements, environmental concerns, ease of use, etc).